Automobile layout

In automotive design, the automobile layout describes where on the vehicle the engine and drive wheels are found. Many different combinations of engine location and driven wheels are found in practice, and the location of each is dependent on the application the vehicle will be used for. Factors influencing the design choice include cost, complexity, reliability, packaging (location and size of the passenger compartment and boot), weight distribution and the vehicle's intended handling characteristics.

Layouts can roughly be divided into two categories: front- or rear-wheel drive. Four-wheel-drive vehicles may take on the characteristics of either, depending on how power is distributed to the wheels.

Front-wheel drive layouts

Front-wheel-drive layouts are those in which the front wheels of the vehicle are driven. The most popular layout used in cars today is the front-engine, front-wheel drive, with the engine in front of the front axle, driving the front wheels. This layout is typically chosen for its compact packaging; since the engine and driven wheels are on the same side of the vehicle, there is no need for a central tunnel through the passenger compartment to accommodate a prop-shaft between the engine and the driven wheels.

As the steered wheels are also the driven wheels, FF (front-engine, front-wheel drive layout) cars are generally considered superior to FR (front-engine, rear-wheel drive layout) cars in conditions such as snow, mud or wet tarmac. However, powerful cars rarely use the FF layout because weight transference under acceleration reduces the weight on the front wheels and reduces their traction, putting a limit on the amount of torque which can be utilized. Electronic traction control can avoid wheelspin but largely negates the benefit of extra torque/power.

A transverse engine (also known as "east-west") is commonly used in FF designs, in contrast to FR which uses a longitudinal engine. The FF layout also restricts the size of the engine that can be placed in modern engine compartments, as FF configurations usually have Inline-4 and V6 engines, while longer engines such as Inline-6 and 90° V8 will rarely fit. This is another reason luxury/sports cars almost never use the FF layout; an exception is the Volvo S80 (FWD/4WD) which uses transversely mounted inline 6 and V8 engines.

Most Audis are FF layout cars, but with longitudinal engines, such as the Audi A4 and Audi A6, however "FrontTrak" front-wheel drive models are only entry-level trims in the United States and Canada; most Audis usually come with "quattro" all-wheel drive. The Audi A3 is a FF layout car with a traverse engine mounting, as it does not share a platform with more expensive offerings in the marque.

Characteristics

Front-wheel drive gives more interior space since the powertrain is a single unit contained in the engine compartment of the vehicle and there is no need to devote interior space for a driveshaft tunnel or rear differential, increasing the volume available for passengers and cargo.[1] There are some exceptions to this as rear engine designs do not take away interior space. (See Porsche 911, and Volkswagen Beetle) It also has fewer components overall and thus lower weight.[1] The direct connection between engine and transaxle reduces the mass and mechanical inertia of the drivetrain compared to a rear-wheel drive vehicle with a similar engine and transmission, allowing greater fuel economy.[1] In front wheel drive cars the mass of the drivetrain is placed over the driven wheels and thus moves the center of gravity farther forward than a comparable rear-wheel drive layout, improving traction and directional stability on wet, snowy, or icy surfaces.[1][2][3] Front-wheel drive cars, with a front weight bias, tend to understeer at the limit, which according to, for instance, Saab engineer Gunnar Larsson, is easier since it makes instinct correct in avoiding terminal oversteer, and less prone to result in fishtailing or a spin.[3][4]

According to a sales brochure for the 1989 Lotus Elan, the ride and handling engineers at Lotus found that "for a given vehicle weight, power and tire size, a front wheel drive car was always faster over a given section of road."[5] However, this may only apply for cars with moderate power-to-weight ratio.[2][6][7] According to road test with two Dodge Daytonas, one FWD and one RWD, the road layout is also important for what configuration is the fastest.[3]

The lack of weight shifting limits the acceleration of a front-wheel drive vehicle. During heavy acceleration, weight is shifted to the back, improving traction at the rear wheels at the expense of the front driving wheels; consequently, most racing cars are rear-wheel drive for acceleration. However, since front-wheel drive cars have the weight of the engine over the driving wheels, the problem only applies in extreme conditions. The weight shifting and weight distribution of rear wheel drive cars make them more likely to oversteer and the related problem of fishtailing. On snow, ice, and sand, rear-wheel drive loses its traction advantage to front or all-wheel drive vehicles which have greater weight over the driven wheels. Rear wheel drive cars with rear engine or mid engine configuration retain traction over the driven wheels, although fishtailing remains an issue. Some rear engine cars (e.g. Porsche 911) can suffer from reduced steering ability under heavy acceleration, since the engine is outside the wheelbase and at the opposite end of the car from the wheels doing the steering. A rear wheel drive car's center of gravity is shifted rearward when heavily loaded with passengers or cargo, which may cause unpredictable handling behavior.[4]

Advantages

Interior space: Since the powertrain is a single unit contained in the engine compartment of the vehicle, there is no need to devote interior space for a driveshaft tunnel or rear differential, increasing the volume available for passengers and cargo.[1]

Cost: Fewer material components and less installation complexity overall. However, the considerable MSRP differential between a FF and FR car cannot be attributed to layout alone. The difference is more probably explained by production volumes as most rear-wheel cars are usually in the sports/performance/luxury categories (which tend to be more upscale and/or have more powerful engines), while the FF configuration is typically in mass-produced mainstream cars. Few modern "family" cars have rear-wheel drive as of 2009, so a direct cost comparison is not necessarily possible. A contrast could be somewhat drawn between the FF Audi A4 and the FR BMW 3-Series, both which are in the compact executive car classification.[1]

Improved drivetrain efficiency: the direct connection between engine and transaxle reduce the mass and mechanical inertia of the drivetrain compared to a rear-wheel drive vehicle with a similar engine and transmission, allowing greater fuel economy.[1]

Assembly efficiency: the powertrain can often be assembled and installed as a unit, which allows more efficient production.

Placing the mass of the drivetrain over the driven wheels moves the centre of gravity farther forward than a comparable rear-wheel drive layout, improving traction and directional stability on wet, snowy, or icy surfaces.[1][2][3]

Predictable handling characteristics: front-wheel drive cars, with a front weight bias, tend to understeer at the limit, which (according to e.g. SAAB engineer Gunnar Larsson) is easier since it makes instinct correct in avoiding terminal oversteer, and less prone to result in fishtailing or a spin.[3][4]

A skilled driver can control the movement of the car even while skidding by steering, throttling and pulling the hand brake (given that the hand brake operates the rear wheels as in most cases, with some Citroen and Saab models being notable exceptions). A small car with the FF layout is superior for motor sport events focusing on manouvreability such as Autotesting[9]

The wheelbase can be extended without building a longer driveshaft (as with rear wheel driven cars).

Disadvantages

Front-engine front-wheel drive layouts are "nose heavy" with more weight distribution forward, which makes them prone to understeer, especially in high horsepower applications. Enthusiast driver aids, such as active rear differential, active steering, and ultra-quick electrically-adjustable shocks, which are found on the 2009 Acura TL-SHAWD and 2010 Audi S4 can negate the understeer problem and allow the car to perform as well as a front-engine rear-wheel drive car (such as the BMW 3 Series). However these trick differentials are heavy, complex, and expensive.[10][11]

Torque steer is the tendency for some front-wheel drive cars to pull to the left or right under hard acceleration. It is a result of the offset between the point about which the wheel steers (it is aligned with the points where the wheel is connected to the steering mechanisms) and the centroid of its contact patch. The tractive force acts through the centroid of the contact patch, and the offset of the steering point means that a turning moment about the axis of steering is generated. In an ideal situation, the left and right wheels would generate equal and opposite moments, canceling each other out; however, in reality, this is less likely to happen. Torque steer can be addressed by using a longitudinal layout, equal length drive shafts, half shafts, a multilink suspension or centre-point steering geometry.[12][13][14][15][16][17][18]

Lack of weight shifting will limit the acceleration of a front-wheel drive vehicle. In a vehicle, the weight shifts back during acceleration, giving more traction to the rear wheels. This is one of the main reasons why nearly all racing cars are rear-wheel drive. However, since front-wheel drive cars have the weight of the engine over the driving wheels, the problem only applies in extreme conditions.

In some towing situations, front-wheel drive cars can be at a traction disadvantage since there will be less weight on the driving wheels. Because of this, the weight that the vehicle is rated to safely tow is likely to be less than that of a rear-wheel drive or four-wheel drive vehicle of the same size and power.

Traction can be reduced while attempting to climb a slope in slippery conditions such as snow- or ice-covered roadways.

Due to geometry and packaging constraints, the CV joints (constant-velocity joints) attached to the wheel hub have a tendency to wear out much earlier than the universal joints typically used in their rear-wheel drive counterparts (although rear-wheel drive vehicles with independent rear suspension also employ CV joints and half-shafts). The significantly shorter drive axles on a front-wheel drive car causes the joint to flex through a much wider degree of motion, compounded by additional stress and angles of steering, while the CV joints of a rear wheel drive car regularly see angles and wear of less than half that of front wheel drive vehicles.

Turning circle — FF layouts almost always use a Transverse engine ("east-west") installation, which limits the amount by which the front wheels can turn, thus increasing the turning circle of a front-wheel drive car compared to a rear-wheel drive one with the same wheelbase. A notable example is the original Mini. It is widely misconceived that this limitation is due to a limit on the angle at which a CV joint can be operated, but this is easily disproved by considering the turning circle of car models that use a longitudinal FF or F4 layout from Audi and (prior to 1992) Saab

The FF transverse engine layout (also known as "east-west") restricts the size of the engine that can be placed in modern engine compartments, so it is rarely adopted by powerful luxury and sports cars. FF configurations can usually only accommodate Inline-4 and V6 engines, while longer engines such as Inline-6 and 90° big-bore V8 will rarely fit, though there are exceptions. One way around this problem is using a staggered engine.

It makes heavier use of the front tires (i.e accelerating, braking, and turning), causing more wear in the front than in a rear wheel drive layout.

Rear wheel drive layouts

FR layout

RR layout

Rear-wheel drive (RWD) typically places the engine in the front of the vehicle and the driven wheels are located at the rear, a configuration known as front-engine, rear-wheel drive layout (FR layout). The front mid-engine, rear mid-engine and rear engine layouts are also used. This was the traditional automobile layout for most of the 20th century.[19] Nearly all motorcycles and bicycles use rear-wheel drive, either by driveshaft, chain, or belt, since the front wheel is turned for steering, and it would be very difficult and cumbersome to "bend" the drive mechanism around the turn of the front wheel.

Characteristics

The vast majority of rear-wheel drive vehicles use a longitudinally-mounted engine in the front of the vehicle, driving the rear wheels via a driveshaft linked via a differential between the rear axles. Some FR layout vehicles place the gearbox at the rear, though most attach it to the engine at the front.

The FR layout is often chosen for its simple design and good handling characteristics. Placing the drive wheels at the rear allows ample room for the transmission in the center of the vehicle and avoids the mechanical complexities associated with transmitting power to the front wheels. For performance-oriented vehicles, the FR layout is more suitable than front-wheel drive designs, especially with engines that exceed 200 horsepower. This is because weight transfers to the rear of the vehicle during acceleration, which loads the rear wheels and increases their grip.

Advantages

Even weight distribution — The layout of a rear-wheel drive car is much closer to an even fore-and-aft weight distribution than a front-wheel-drive car, as more of the engine can lie between the front and rear wheels (in the case of a mid engine layout, the entire engine), and the transmission is moved much farther back.[20]

Weight transfer during acceleration — During heavy acceleration, weight is placed on the rear, or driving wheels, which improves traction.

No torque steer[21] (unless it's an all-wheel steer with an offset differential).

Steering radius — As no complicated drive shaft joints are required at the front wheels, it is possible to turn them further than would be possible using front-wheel drive, resulting in a smaller steering radius for a given wheelbase.

Better handling in dry conditions — the more even weight distribution and weight transfer improve the handling of the car. The front and rear tires are placed under more even loads, which allows for more grip while cornering.[22]

Towing — Rear wheel drive puts the wheels which are pulling the load closer to the point where a trailer articulates, helping steering, especially for large loads.[23]

Serviceability — Drivetrain components on a rear-wheel drive vehicle are modular and do not involve packing as many parts into as small a space as does front wheel drive, thus requiring less disassembly or specialized tools in order to service the vehicle.

Robustness — due to geometry and packaging constraints, the universal joints attached to the wheel hub have a tendency to wear out much later than the CV joints typically used in front-wheel drive counterparts. The significantly shorter drive axles on a front-wheel drive car causes the joint to flex through a much wider degree of motion, compounded by additional stress and angles of steering, while the CV joints of a rear wheel drive car regularly see angles and wear of less than half that of front wheel drive vehicles.

Disadvantages

On snow, ice and sand, rear-wheel drive loses its traction advantage to front- or all-wheel drive vehicles, which have greater weight on the driven wheels. Rear-wheel-drive cars with rear engine or mid engine configuration do not suffer from this, although fishtailing remains an issue.

Some rear engine cars (e.g. Porsche 911) can suffer from reduced steering ability under heavy acceleration, because the engine is outside the wheelbase and at the opposite end of the car from the wheels doing the steering although the engine weight over the rear wheels provides outstanding traction and grip during acceleration.

Decreased interior space — Though individual designs vary greatly, rear wheel drive vehicles may have: Less front leg room as the transmission tunnel takes up a space between the driver and front passenger, less leg room for center rear passengers (due to the tunnel needed for the drive shaft), often no seat for a center rear passenger, and sometimes less boot space (since there is also more hardware that must be placed underneath the boot). Rear engine designs (such as the Porsche 911 and Volkswagen Beetle) do not inherently take away interior space.

Increased weight — The components of a rear wheel drive vehicle's power train are less complex, but they are larger. The driveshaft adds weight. There is extra sheet metal to form the transmission tunnel. There is a rear axle or rear half-shafts, which are typically longer than those in a front-wheel drive car. A rear wheel drive car will weigh slightly more than a comparable front wheel drive car (but less than four wheel drive).

Improper weight distribution when loaded — A rear wheel drive car's center of gravity is shifted rearward when heavily loaded with passengers or cargo, which may cause unpredictable handling behavior.[4]

Higher initial purchase price — Modern rear wheel drive vehicles are typically more expensive to purchase than comparable front wheel drive vehicles. Part of this can be explained by the added cost of materials and increased complex assembly of FR layouts, as the powertrain is not one compact unit. However, the difference is more probably explained by production volumes as most rear-wheel cars are usually in the sports/performance/luxury categories (which tend to be more upscale and/or have more powerful engines), while the FF configuration is typically in mass-produced mainstream cars.

The possibility of a slight loss in the mechanical efficiency of the drivetrain (approximately 17% coastdown losses between engine flywheel and road wheels compared to 15% for front wheel drive — however these losses are highly dependent on the individual transmission). Cars with rear engine or mid engine configuration and a transverse engine layout do not suffer from this.

The long driveshaft (on front engine cars) adds to drivetrain elasticity.[4] The driveshaft must also be extended for cars with a stretched wheelbase (e.g. limousines, minivans).

When shifting down at high RPM, rear-wheel drive vehicles tend to oversteer if the clutch is not released correctly.

Four wheel drive layouts

Front-engine, rear-wheel drive derived “F4” layout

Note: in North America, Australia and New Zealand the term "four-wheel drive" usually refers only to drivetrains which are primarily two-wheel drive with a part-time four-wheel drive capability, as typically found in pickup trucks and other off road vehicles, while the term "all-wheel drive" is used to refer to full time four-wheel drive systems found in performance cars and smaller car-based SUVs. This section uses the term four-wheel drive to refer to both.

For a full explanation of 4WD engineering considerations, see the main article on four-wheel drive

Advantages

In terms of handling, traction and performance, 4WD systems generally have most of the advantages of both front-wheel drive and rear-wheel drive. Some unique benefits are:

Traction is nearly doubled compared to a two-wheel drive layout. Given sufficient power, this results in unparalleled acceleration and driveability on surfaces with less than ideal grip, and superior engine braking on loose surfaces. The development of 4WD systems for high performance cars was stimulated primarily by rallying.

Handling characteristics in normal conditions can be configured to emulate FWD or RWD, or some mixture, even to switch between these behaviours according to circumstance. However, at the limit of grip, a well balanced 4WD configuration will not degenerate into either understeer or oversteer, but instead break traction of all 4 wheels at the same time into a four-wheel drift. Combined with modern electronic driving aids, this flexibility allows production car engineers a wide range of freedom in selecting handling characteristics that will allow a 4WD car to be driven more safely at higher speeds by inexpert motorists than 2WD designs.

Disadvantages

4WD systems require more machinery and complex transmission components, and so increase the manufacturing cost of the vehicle and complexity of maintenance procedures and repairs compared to 2WD designs

The handbrake cannot be used to induce over-steer for maneuvering purposes, as the drivetrain couples the front and rear axles together. To overcome this limitation, some custom prepared stage rally cars have a special mechanism added to the transmission to disconnect the rear drive if the handbrake is applied while the car is moving.

Unusual 4WD layouts

From 1989 onwards, some models of Porsche 911 feature a rear-engined 4WD layout, which is akin to a longitudinal front-engine 4WD layout installed backwards with the engine at the rear of the car

From 2007 onwards, the Nissan GT-R features a front-engine 4WD longitudinal layout, but with the gearbox at the rear of the vehicle. This provides a more ideal weight balance, and improves directional stability at very high speeds by increasing the vehicle's moment of inertia around the vertical axis. This layout necessitates a second prop-shaft to carry power to the front wheels.

Some types of farm tractors and construction site machinery use a 4WD layout where the wheels on each side are coupled together, rather than the wheels on each axle, allowing these vehicles to pivot about their center point. Such vehicles are controlled in a fashion similar to a military tank.

The Citroën Sahara had a 4WD system using complete Citroën 2CV drivetrains at both ends of the car, such that the engine at the front powered the front wheels and the engine at the back powered the rear wheels.[24]

History and current use

FMR layout, standard in most Front-engine / Rear-wheel drive cars pre-World War II, where the engine was located behind the front axle.

The first FR car was an 1895 Panhard model, so this layout was known as the "Système Panhard" in the early years. Most American cars used the FR layout until the mid 1980s. The Oil crisis of the 1970s and the success of small FF cars like the Mini, Volkswagen Golf, Toyota Tercel, and Honda Civic led to the widespread adoption of that layout.

After the Arab Oil Embargo of 1973 and the 1979 fuel crises, a majority of American FR vehicles (station wagons, luxury sedans) were phased out for the FF layout — this trend would spawn the SUV/van conversion market. Throughout the 1980s and 1990s, most American companies set as a priority the eventual removal of rear-wheel drive from their mainstream and luxury lineup.[25] Chrysler went 100% FF by 1990 and GM's American production went entirely FF by 1997 except the Corvette and Camaro. Ford's full-size cars (the Ford Crown Victoria, Mercury Grand Marquis, and Lincoln Town Car) have always been FR,[26] as was the Ford Mustang[27] and Lincoln LS. In 2008 Hyundai introduced its own rear-wheel drive car, the Hyundai Genesis.

In Australia, FR cars have remained popular throughout this period, with the Holden Commodore and Ford Falcon having consistently strong sales. In Europe, front-wheel drive was popularized by small cars like the Mini, Renault 5 and Volkswagen Golf and adopted for virtually all mainstream cars.

Upscale marques like Mercedes-Benz, BMW, and Jaguar remained mostly independent of this trend, and retained a lineup mostly or entirely made up of FR cars.[28] Japanese mainstream marques such as Toyota and Nissan became mostly or entirely FF early on, while reserving for their latterly-conceived luxury divisions (Lexus and Infiniti, respectively) a mostly FR lineup. While many automakers lost sight of the true sports car, Mazda introduced the highly successful Miata roadster in 1990, a true 2-seater sports car using the traditional FR layout which led to other compaines such as General Motors to produce a FR sports car based on their Kappa platform.

Currently most cars are FF, including virtually all front-engined economy cars, though FR cars are making a return as an alternative to large sport-utility vehicles. In North America, GM returned to production of the FR luxury car with the 2003 Cadillac CTS, and with the removal of the DTS,[29] Cadillac will be entirely FR (with four-wheel drive available as an option on several models) by 2010, and the 2010 Camaro returns as a FR sports car. Chrysler returned its full-size cars to this layout with the Chrysler 300 and related models.[30][31] Ford never eliminated FR cars, but is looking to replace the dated designs that it currently has.[32] Nissan are also bringing back the Silvia to their line-up, Mazda is said to be releasing a new rotary-powered FR car in their RX line-up by 2010 and there are rumours Toyota are bringing a successor to the AE86, in other words, an affordable RWD car. Hyundai introduced their affordable RWD car being the 2009 Hyundai Genesis and 2010 Hyundai Genesis Coupe

In the 21st century, with solutions to the engineering complexities of 4WD being widely understood, and consumer demand for increasing performance in production cars, front-engined 4WD layouts are rapidly becoming more common, and most major manufacturers now offer 4WD options on at least some models. Manufacturers with a notable expertise and history in producing 4WD performance cars are Audi and Subaru.

See also

References

↑ 2.02.12.2William, Milliken (1995). "Merits of Front-, Rear-, and Four-Wheel Drive". Race Car Vehicle Dynamics. SAE International. p. 730. ISBN1560915269. "Front-wheel drive has been most successful in the lower power/weight range and in sutuations in which superior derectional stability on low coefficients is important. There has never been a successful front-drive Grand Prix car nor a competitive Indianapolis car of more than 300 hp."

↑Prost, Alain (1990). "Controlling a car at the limit". Competition Driving. Hazelton Publishing. p. 50pp. ISBN0905138805. "Front-wheel drive. In this instance, both power and steering are directed through the front wheels, the rears remaining free. Following the principle of weight transfer once more, the lightening of the front wheels under acceleration considerably reduces their effectiveness and thus limits the usable power. Consequentally, this type of transmission is generally less effective on racing circuits, a few rare exceptions notwithstanding, but has its advantages in road events where maximum power is not called into play so often"